A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or a hydrocarbon (e.g., methanol), to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cells employing methanol as the fuel supplied to the anode (also commonly referred to as a “Direct Methanol Fuel Cell (DMFC)” system), the electrochemical reactions are essentially as follows: first, a methanol molecule's carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule's oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air or oxidant (supplied to the cathode) is likewise simultaneously reduced at the cathode. The ions (protons) formed at the anode migrate through the interposing electrolyte and combine with the oxygen at the cathode to form water. From a molecular perspective, the electrochemical reactions occurring within a direct methanol fuel cell (DMFC) system are as follows:
                    Anode        :                                                                CH              3                        ⁢            OH                    +                                    H              2                        ⁢            O                          →                              6            ⁢                          H              +                                +                      6            ⁢                                                  ⁢                          e              -                                +                      CO            2                                                        E          0                =                  0.04          ⁢                                          ⁢          V                                    vs        .                    NHE                      (        1        )                                Cathode        :                                                                                3                2                            ⁢                              O                2                                      +                          6              ⁢                              H                +                                      -                          6              ⁢                              e                -                                              →                      3            ⁢                          H              2                        ⁢            O                          ⁢                                                                E          0                =                  1.23          ⁢                                          ⁢          V                                    vs        .                    NHE                      (        2        )                                Net        :                                                                CH              3                        ⁢            OH                    +                                    3              2                        ⁢                          O              2                                      →                              2            ⁢                          H              2                        ⁢            O                    +                      CO            2                                                        E          0                =                  1.24          ⁢                                          ⁢          V                                    vs        .                    NHE                      (        3        )            
The various electrochemical reactions associated with other state-of-the-art fuel cell systems (e.g., hydrogen or carbonaceous fuel) are likewise well known to those skilled in the art of fuel cell technologies.
With respect to state-of-the-art fuel cell systems generally, several different configurations and structures have been contemplated—most of which are still undergoing further research and development. In this regard, existing fuel cell systems are typically classified based on one or more criteria, such as, for example: (1) the type of fuel and/or oxidant used by the system, (2) the type of electrolyte used in the electrode stack assembly, (3) the steady-state operating temperature of the electrode stack assembly, (4) whether the fuel is processed outside (external reforming) or inside (internal reforming) the electrode stack assembly, and (5) whether the reactants are fed to the cells by internal manifolds (direct feed) or external manifolds (indirect feed). In general, however, it is perhaps most customary to classify existing fuel cell systems by the type of electrolyte (i.e., ion conducting media) employed within the electrode stack assembly. Accordingly, most state-of-the-art fuel cell systems have been classified into one of the following known groups:                1. Alkaline fuel cells (e.g., KOH electrolyte);        2. Acid fuel cells (e.g., phosphoric acid electrolyte);        3. Molten carbonate fuel cells (e.g., Li2CO3/K2CO3 electrolyte);        4. Solid oxide fuel cells (e.g., yttria-stabilized zirconia electrolyte);        5. Proton exchange membrane fuel cells (e.g., NAFION electrolyte).        
Although these state-of-the-art fuel cell systems are known to have many diverse structural and operational characteristics, such systems nevertheless share many common characteristics with respect to their electrode structures. For example, one type of common electrode structures consists essentially of a conductive substrate (e.g., metal plate or porous carbon-fiber sheet) that has a substantially planar catalytic film/layer thereon (e.g., affixed or embedded catalysts particles). Another type of electrode structure consists essentially of a porous bulk matrix substrate (e.g., silicon and/or sol-gel) that has catalyst particles chemisorbed on the pore surfaces (see, e.g., International Publication No. WO 01/37357, which publication is incorporated herein by reference in its entirety). Some of the problems associated with existing porous electrode structures include, for example: (1) poor catalyst utilization, (2) less than optimal electrical conductivity, and (3) mass transfer limitations associated with reactants reaching catalytic electrode surfaces. Thus, there is still a need in the art for improved fuel cell electrode structures, assemblies, and systems. The present invention fulfills these needs and provides for further related advantages.